In aircraft, chip heaters may be used to provide anti-icing in vanes and pitot probes, and these chip heaters may exhibit changes in resistance with changes in applied voltage. When an AC voltage is applied, this effect may lead to a current draw that is not a constant proportion of the applied voltage. The frequency content of the voltage therefore may not match the frequency content of the current, which may lead to harmonic products in the current waveform that are not present in the supplied voltage. Aircraft manufacturers often specify the maximum amplitude of the current harmonics, based on the capabilities of the power generation devices on the particular aircraft, and the harmonics produced in chip heaters are often outside the normally required range. The closer the current is to an exact, constant proportion (multiple) of the voltage, the lower the harmonic products.